CHAPTER IV 



MEIOSIS IN DIPLOIDS AND POLYPLOIDS 



Introduction — Outline of Meiosis — Meiosis in Diploids — Meiosis in Poly- 

 ploids — Forms of Bivalents and Multivalents at Metaphase — Separation at 

 Anaphase — Conclusion — The Conditions of Chromosome Pairing. 



I. INTRODUCTION 



There must be a form of nuclear division in which the ancestral germ 

 plasms contained in the nucleus are distributed to the daughter-nuclei in 

 such a way that each of them receives only half the number contained in the 

 original nucleus. 



Weismann, 1887. 



Studies of fertilisation between 1870 and 1875 showed that the 

 process consisted essentially in the union of the nuclei of the fusing 

 germ cells (O. Hertwig). Studies of mitosis during the succeeding 

 years showed that at every observed division of the nucleus or 

 mitosis there was a division of its constituent elements, the 

 chromosomes, into equal halves. These elements were constant 

 in appearance, and therefore they were, as later work has abundantly 

 shown, " permanent " structures. 



It follows that, in the history of the nucleus from one generation 

 to the next, wherever fertilisation occurs, there must also be some 

 compensating process. Addition must be set off by reduction. 

 This conclusion was arrived at by Weismann in 1887, although at 

 that time the nature of the reduction was only slightly indicated by 

 observations of germ-cell formation on the female side. Weismann 

 noted that in certain parthenogenetic organisms only one polar 

 body was extruded instead of two as in sexual eggs. He thought, 

 therefore, that the division which yielded the second polar body 

 was a " reducing division " — a conclusion that was sound in a 

 certain sense. He further concluded that a similar reduction would 

 be found in male germ-cell formation. 



Weismann's induction has been verified universally : wherever 



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